The use of band-limited voice channels for the transmission of high-speed data signals has been made possible in recent years by the development of a range of modems which incorporate data receivers that compensate for the major impairing effects encountered on such channels. These high-speed modems generally make use of two-dimensional modulation techniques whereby groups of binary data for transmission are encoded into two-dimensional symbols (made up of `in-phase` and `quadrature` components) which are then transmitted by amplitude modulating two quadrature carrier signals. The closed set of two-dimensional data symbols characterizes the type of modulation scheme; typical examples are phase shift keying (PSK) and quadrature amplitude modulation (QAM). The two-dimensional symbols can conveniently be represented on a two-dimensional diagram (known in the art as a `constellation` diagram) with the in-phase and quadrature components of each symbol being measured off along respective orthogonal axes. FIG. 1 of the accompanying drawings illustrates a `16-QAM` constellation where each symbol represents four binary bits of a data signal being transmitted.
The constellation diagram shown in FIG. 1 is, of course idealized. After the symbols have been transmitted over a voice-frequency data channel, the constellation diagram of the received symbols shows various distortions due to the effect of a range of quasi-static (or basically steady-state) and transient channel impairments.
Of the quasi-static impairments, linear distortion caused by the band-limiting filter action of the channel is potentially the most troublesome and manifests itself as interference between adjacent symbols in the received data signal. Such intersymbol interference (ISI) is normally minimized by means of an automatic adaptive equalizer incorporated within the data receiver. Without such equalization, however, the effect of channel linear distortion would be seen on a constellation diagram as a `cloud of uncertainty` clustered around each constellation point (a similar effect is produced by additive background noise which co-exists with the data signal in the same spectral band). It should also be noted that as well as linear distortions, the channel will generally introduce second and third order non-linear distortions.
Two further significant quasi-static impairments are frequency offset and phase jitter which manifest themselves as a time-dependent variation in the reference phase of the data signal carrier wave. To minimize these effects, the data receiver normally includes a decision-directed phase-locked loop (PLL) which tracks variations in the reference phase of the data signal carrier wave. Without such a tracking device, the effects of frequency offset would be seen as a slow rotation of the constellation points about their origin, whilst the effect of phase jitter would be seen as an angular oscillation of the constellation points.
Another quasi-static impairment met in band-limited voice channels is amplitude jitter which manifests itself as a time-dependent variation in the level of the received data signal. It is possible to compensate for the effect of amplitude jitter by including within the data receiver a decision-directed gain-control-loop which tracks variations in the level of the received signal. Without such a control mechanism, however, the effect of amplitude jitter would be seen on the data constellation diagram as a radial oscillation of the constellation points.
A known method of measuring channel impairments is to apply tones of predetermined frequency and amplitude to the channel and then observe the signal received at the far end. This method has the considerable disadvantage of requiring an interruption in the normal channel traffic.